Kirk is used as a pawn to carry disease to a planet cursed with long life and over-population.
When I look at a program, I ask two questions. 1 - Is the story grand or unique and does it say something powerful about people and the human condition? Is some of the dialog unforgettable? 2 - Is the script competent and believable? If one fails and the other doesn't, it still has some merit to me. "The Mark of Gideon" fails both tests. Hard.
The planet dynamics make no sense, no world could be so populated that it teems on every surface with one species. If one accepts the premise, why would birth control be so repugnant? The writers struggle with this by putting in non-sequiters that essentially say that the people love life so much that they long to die. How in the world can Gideon be capable of making a fake Enterprise so precisely that it simulates every single function of the real ship but they can't figure out how to make the view screens any better than cheap one-way glass? Other episodes make a strong point that one man cannot pilot the Enterprise, Kirk seems unconcerned about it. Aside from some of Spock's dialog with the planet, the writing quality is not sterling here either. Kirk says goodbye to his love interest with, "you are needed everywhere." Good grief, THAT'S on-topic.
Aside from wanting to deal with a serious issue like over-population and 30 minutes of dramatic mystery (what the fake Enterprise is), this episode could contend for the worst of the worst. It may be in the bottom five.